Add a missing include. This is important in the case HandleLLVMOptions is
included prior to the missing CheckCXXSourceCompiles or CheckCXXCompilerFlag
which includes CheckCXXSourceCompiles.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@262949 91177308-0d34-0410-b5e6-96231b3b80d8
Otherwise users get messages from CheckAtomic about missing libatomic
instead of a sensible message that says "use GCC 4.7 or newer".
I structured the change along the lines of HandleLLVMStdlib.cmake, so
that the standalone build of Clang still gets the compiler version
check.
Reviewers: beanz
Differential Revision: http://reviews.llvm.org/D17789
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@262491 91177308-0d34-0410-b5e6-96231b3b80d8
